3
Problems with JPL Scheme 30kb/s is comparable to current radio systems Sensitivity falls far short of quantum limit –Problems with analogue APD detection. –Large detector size required to match telescope. –Background from daylight sky and Mars. 10m telescopes are expensive - and need several of them. Operating when Mars is near the Sun impossible.

5
Coherent Detection Mix input signal with local oscillator and detect at the beat frequency. Quantum limited performance - e.g. 4.5 photons/bit demonstrated in ESA experiments. Not suitable for large ground based telescopes. Good for space to space systems.

9
Advantages Overall telescope is compact. No need to make a large mirror. No problems matching detector size. No need for active control systems. Long (f/5) tube assemblies can be baffled to allow operation within ~10 degrees of Sun. Redundancy. Scalable to any size you want.

10
Daylight Operation Essential for system to be able to operate in daylight. –To follow Mars throughout its orbit. –Background from Mars itself will be at daylight levels. Need narrow band filter with width ~10 –6 –Tunable to follow orbital motion of spacecraft –Can be achieved by Fabry-Perot and probably other technologies.